1. Technical Field
This invention relates in general to telecommunications and, more particularly, to a system and device for locating a mobile transceiver.
2. Description of the Related Art
Mobile communications has demonstrated enormous growth over the last decade. Whereas mobile communications was once limited to critical phone calls in situations where wireline communications did not exist, many people now use mobile communications as their primary means of communicating. Further, many present day mobile communications include processing capabilities, such as PDAs (personal digital assistants), hand-held computers, notebook computers, and other devices. Some pagers also have two way communication capabilities.
At times, it is desirable to locate a mobile communication device with some precision. An obvious example of an application where mobile locating services are beneficial is in connection with emergency (i.e., 911) services. In emergency situations, the user of a mobile device may be unaware or his or her location, or in a stressed state where he or she is confused about the present location. This can result in critical emergency services, such as police, fire and ambulance services, being sent to a wrong location.
Other services may also need location information for optimal efficiency. For example, a company may provide xe2x80x9croadside assistancexe2x80x9d or xe2x80x9cconciergexe2x80x9d services to customers for a fee. These services could provide directions to the nearest gas station, towing services, directions and so on for users of mobile devices. To effectively administer the services, it will be desirable to have fairly accurate information on the user""s location.
One solution would be to provide a GPS (global positioning system) device in each phone, which could send location information as part of a request for services. While a GPS device can produce extremely accurate location information, it requires clear reception of signals from multiple satellites. Typically, the satellite signals cannot be accurately received while the mobile communications device is within a building, or while tall buildings surround the mobile communications device. Accordingly, the GPS location services will be inoperable in many circumstances where they are needed the most.
A number of solutions use triangulation to determine the location based on a transmission from the mobile device. These solutions include TOA (time of arrival) and TDOA (time difference of arrival) techniques. Time synchronization is of particular importance for the receivers (typically, base stations) receiving the signal, since even small variations (as low as tens of nanoseconds) can cause substantial inaccuracies in the resultant location calculation.
Therefore, a need has arisen in the industry for a highly accurate method and apparatus for determining the location of a mobile communications device.
In the present invention, a system for locating one of a plurality of mobile communications devices is provided. A transmitter transmits a first signal, with the first signal having a device identification code for identifying a particular mobile communications device to be located and a time indicator. Circuitry in each of the mobile communications devices receives the first signal and transmits a second signal in response to a match between the identification code of the first signal and an internal identification code. Each of a plurality of receivers has an internal clock: When the first signal is received, these clocks are set based on the time indicator in the first signal. When the second signal is received, a time difference between the first signal and second signal is determined. Control circuitry determines the location of the particular mobile communications device responsive to time differences determined by multiple receivers.
The present invention provides significant advantages over the prior art. First, extremely accurate location information can be derived. Second, the signals may be transmitted using conventional technology usable within buildings. Third, the effect of propagation delays within the receiver is eliminated. Fourth, only three receivers are needed to unambiguously locate the mobile communications device, although more receivers may provide greater accuracy.